2. When waves encounter obstacles, the bending
of waves around the edges of an obstacle is cal
3. Huygen’s principle requires that
the waves spread out after
they pass through Slits.
This spreading out of light
from its initial line of travel
is called diffraction
In general, diffraction occurs
when wave pass through
small openings, around
obstacles or by sharp edges
4. Here we have a source s emitting waves having
As the wavefronts pass through the slit ab, they
are diffracted and are able to reach even those
regions behind ab which they would be unable
to reach had the rays not bended.
One more thing to be noted here is that the
shape of the wavefronts change as they pass
through the slit.
The reason for this bending can be explained
with the help of huygens principle.
6. Huygen’s principle:
Each particle lying on any wavefront acts as an independent
secondary source and emits from itself secondary spherical
waves. After a very small time interval, the surface tangential to
all these spherical wavelets, gives the position and shape of the
It will be more clear from the following example of plane
Let p1,p2,p3,…pn be points very close to each other and
equidistant from each other on the plane incident wavefront.
7. To obtain the new wavefront we consider p1,p2,..Pn as
independent sources and circular arcs with same radius
from each of these points.
Now the plane a’ tangential to all these imaginary surfaces
gives the new wavefront.
We move on to using huygen’s principle for explanation of
The dimensions of the slit are finite. As a result, applying
huygen’s principle we can say that the new wavefront
obtained will be something.
Relation of Fresnel diffraction to Fraunhofer diffraction by a single slit
11. When the distance between the slit ab and so
urce of light s as well as between slit ab and t
he screen is finite, the diffraction is called Fre
In Fresnal diffraction the waves are either sph
erical or cylindrical.
12. If light incident on slit ab is coming from inf
inite distance, the distance between obstacl
e a and screen c is infinite, the diffraction is
called Fraunhofer diffraction.
In Fraunhofer diffraction the incident waves
should have plane wavefronts.
13. X-rays have wavelengt
hs comparable to atom
ic sizes and spacings,
about 10–10 m
Crystals and molecules
reflect X-rays in specifi
c patterns depending o
n their structures X-ray diffraction pattern of myoglobin
16. Bragg’s Law
• W. H. Bragg and W. L. Bragg, 1913 (Nobel 1915)
• Condition for constructive interference:
2dsinθ = nλ
• Diffraction from different sets of planes in the cr
ystal gives a picture of the overall structure
17. We have seen how we can get an interferenc
e pattern when there are two slits. We will
also get an interference pattern with a sing
le slit provided it’s size is approximately l
(neither too small nor too large)
18. To understand single slit diffraction, we must consider e
ach point along the slit (of width a) to be a point sourc
e of light. There will be a path difference between ligh
t leaving the top of the slit and the light leaving the mi
ddle. This path difference will yield an interference pat
Path difference of rays to P from top and bottom edge of
DL = a sinq destructive if
DL = ml,
Notice that central maxim
um is twice as wide as
Sinq = m l / W, Destructi
Dark Fringes on screen
y = L tanq L (ml/W)
Maxima occur for y= 0 an
y L (m1/2)(l/W)